The invention relates to a rotary anode for an X-ray tube, having a support made of carbon, a target layer made of a heavy metal and a rhenium-containing intermediate layer comprising several sub-layers sandwiched between the support and the target layer.
The support of the rotary anode consists, for example, of graphite, particularly electrographite, of pyrolytic graphite or of foamed carbons as described in German Offenlegungsschrift No. 2,453,204 and German Offenlegungsschrift No. 2,648,900. The support may alternatively be composed of sub-elements of these materials, for example electrographite or pyrolytic graphite.
In the literature the target layer is also referred to as the electron bombardment area (DE-PS No. 2,115,896), X-ray active layer, anti-cathode or collision electrode layer (DE-OS No. 2,748,566). It consists of, for example, tungsten, molybdenum, tantalum or alloys of these metals with one another or with rhenium.
AT-PS No. 281,213 corresponding to British Pat. No. 1,247,244 discloses a rotary anode in which a rhenium intermediate layer is arranged between the graphite support and the tungsten or tungsten-alloy target layer. The tungsten alloy can be, for example, a tungsten-osmium or a tungsten-iridium alloy. Diffusion of the graphite into the target layer is almost completely prevented by this intermediate layer. During the investigations which resulted in the invention it was found, however, that, above 1500 K. the desired antidiffusion effect is only obtained for a sufficient period of time with intermediate rhenium layers having a thickness of several tens of .mu.m. Such layers are quite expensive.
In the rotary anode described in DE-OS No. 2,748,566 an intermediate layer containing rhenium and molybdenum is sandwiched between the graphite support and the target layer consisting of tungsten or of a tungsten alloy. The intermediate layer is composed of two sub-layers, the sub-layer which contacts the support containing a large quantity of rhenium, for example 60 to 90% by weight of this sub-layer consists of rhenium, whereas the sub-layer which contacts the target layer contains a large quantity of molybdenum. Molybdenum-containing intermediate layers have indeed a very good adhesion. However, at temperatures above 1500 K. molybdenum combines with the graphite of the support to form molybdenum carbide which has a relatively poor heat conductivity and which furthermore affects the adhesion between the target layer, which, for example, consists of tungsten, and the graphite support, so that the target layer may become wholly detached from the support when it is loaded by an electron beam for a prolonged period of time.